The present invention relates methods and apparatus for moving liquids, particularly volatile liquids, from one, vessel to another. In more detail, the present invention relates to methods and apparatus for balancing the vapor pressure of two liquid vessels, such as a storage tank and a railroad car, during the movement of the liquid from one vessel to the other so as to increase the rate of fluid movement and decrease vapor loss, for depressurizing the vessel from which the liquid is pumped, and for assisting with the recovery of vent gas from the vent gas recovery system of the facility.
Equipment is disclosed in the patent literature, and a number of systems are currently in use, for loading and unloading liquids from, for instance, a barge to a storage tank or a storage tank to a tank truck. U.S. Pat. Nos. 3,527,384 and 4,014,358 describe equipment and methods for moving liquids from one container to another. Of particular concern in the movement of such liquids are combustible and/or volatile liquids, and the patent literature includes, for instance, U.S. Pat. Nos. 3,783,911 and 5,125,439, directed to methods for collecting and/or controlling vapor emissions during loading and unloading of such liquids. It is also known in the art to inject natural gas or propane into the ullage of the tank from which the liquid is pumped to balance the tanks and then to flare off that gas to depressurize the tank.
In addition to the flammability of the vapors and/or the gas used to balance the tanks during loading and unloading, such systems are complicated by the distance the liquid must be moved and/or the physical requirements of the facility at which they are being moved. For instance, it is not uncommon to have a loading/unloading facility at which the liquid must be moved from a barge to a storage tank that may be several hundred, or even several thousand, feet away from the barge. Further, for safety and other reasons, the pumps and electrical equipment for moving the liquid may be located remotely from the barge. In short, the design of such systems is almost always constrained by the specifics of each installation.
There is, therefore, a need for methods and apparatus which are adaptable for moving liquids from one container to another for use in many locations, regardless of the distance the liquid must be transported and the available electrical capacity and proximity. There is also a need for methods and apparatus which are safely used for moving combustible and highly volatile liquids from container to container. There is also a need for methods and apparatus which increase the speed with which liquids are moved from one container to another. There is also a need to reduce facility-wide vented vapors which must be separately compressed, condensed and piped to storage tanks.
These needs, and others known to those skilled in the art, are met by providing an apparatus for balancing vapor while moving liquid from a first vessel to a second liquid storage vessel comprising a pump for unloading or loading liquid from a first vessel to a second vessel, a vapor source, and a vapor line in fluid communication with the vapor source and the first vessel for returning vapor to the first vessel to replace the liquid pumped from the first vessel to maintain pump suction pressure. A separator with a heating coil and a compressor is provided for pulling vapor off the then-unloaded second vessel as is a condenser (which is in fluid communication with the compressor and the second vessel) for changing vapor from the compressor to liquid and then routing the liquid to the second vessel.